This invention relates generally to an improved method for the manufacture of an optical recording medium on which information can be read or written by a laser beam and the optical recording medium produced by such method. In particular, the improved method of the present invention involves the adhering together of first and second substrates, at least one of which includes an information recording layer, under partial vacuum conditions.
Generally, a conventional optical recording medium (ORM) includes an information recording layer disposed intermediate first and second substrates. To construct such a conventional optical recording medium, the information recording layer is formed on one of the first or second substrates, an adhesive material is applied to the surface of one of the substrates and then the other substrate is pressed against the adhesive on the first substrate to bond the substrates together. Conventional methods use a rollcoater or sprayer to apply the adhesive under normal atmospheric pressure. Force is then applied to press the substrates together under normal atmospheric pressure to insure that a proper bond is formed.
Reference is made to FIG. 31 of the drawings which illustrates the conventional method for manufacturing an optical recording medium using a first substrate 91 and a second substrate 93. An adhesive layer 92 is applied intermediate first substrate 91 and second substrate 93. Force is then applied to press the substrates together by a weight 90 under normal atmospheric pressure to insure proper contact. When an optical recording medium is constructed in this manner, air bubbles are inevitably formed in adhesive layer 92 thereby causing defects in the optical recording medium and producing errors in the information read from the information recording layer. Additionally, the air bubbles in the adhesive layer cause the dielectric layer, which is used to protect the information recording layer, to crack thereby causing burst errors and bit errors.
When force is used to adhere the substrates and to improve dimensional accuracy, it is very difficult to apply the force uniformly across the entire substrate. Non-uniform force causes birefringence in both the substrates and the adhesive layer. Additionally, dust on the pressing plate is forced against the substrate surface thereby causing defects such as unevenness in the substrate surfaces.
Another conventional method for producing substrates includes the use of injection molded plastic base substrates as shown in FIG. 32. Such base substrates have variations in the length and in the thickness of the outer periphery of the substrates. When such injection molded substrates are adhered together with force, surface deflection and warp of the adhered substrates tends to be greater than it is in a single substrate.
Due to these problems, conventional construction methods will not produce a high quality optical recording medium with a high yield. Accordingly, it is desired to provide an improved optical recording medium and an improved method for manufacturing an optical recording medium.